(1) Field of the Invention
The present invention relates to a process for the continuous preparation of a methacrylimide-containing polymer having an excellent transparency and heat resistance.
(2) Description of the Related Art
Since methacrylic resins have excellent transparency, weatherability, and mechanical properties, they are used as high-performance optical materials, decorative materials and parts of automobiles and electric appliances, and applications of these resins to the field of short-distance optical communication and as light sensors have been tried and developed. However, since the heat distortion temperature of methacrylic resins is low, about 100.degree. C., they cannot be easily used in fields where a high heat resistance is required, and therefore, it is eagerly desired to improve the heat resistance of methacrylic resins.
As a method for improving the heat resistance of methacrylic resins, there is known methods in which a methyl methacrylate polymer is imidized. For example, there have been proposed (1) method in which a polymer of acrylic acid, methacrylic acid or an ester thereof is heated and reacted with a primary amine, ammonia or a compound capable of forming ammonia or a primary amine in the presence of a solvent (see U.S. Pat. No. 2,146,209), (2) a method in which a polymer of methyl methacrylate is reacted with a primary amine in the presence of water (see U.S. Pat. No. 3,284,425), and (3) a method in which an acrylic polymer is reacted with ammonia or a primary amine in an extruder (see U.S. Pat. No. 4,246,374).
Since the boiling point of the solvent used in the method (1) is high, it is difficult to completely separate the solvent from the polymer formed by the imidization on a commercial scale, and therefore, the formed polymer is colored and the transparency is degraded. In the method (2), since the reaction is carried out in the presence of water, if it is intended to obtain a partially imidized polymer, hydrolysis occurs in the methyl methacrylate segments and it is difficult to obtain an imidized polymer having an intended high heat resistance. Furthermore, in the method (2), it is difficult to conduct the imidization reaction uniformly. In the method (3), since the imidization reaction is effected between a highly viscous polymer and a gaseous imidizing substance, it is difficult to perform the imidization reaction uniformly, and therefore, a uniformly partially imidized polymer cannot be easily obtained.
Even though the heat resistance is improved in imidized polymers obtained according to the foregoing known methods, if polymers are prepared on a commercial scale, the transparency is degraded, the molecular weight is substantially reduced, or the imidization reaction is uneven. Accordingly, the foregoing known methods are not practically workable.